1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image by discharging liquid from a liquid discharge head onto a recording medium.
2. Description of Related Art
In conventional general image forming apparatus, an image is formed on a recording medium by discharging liquid such as ink and treatment liquid for adjusting a fixing ability of the ink from discharge ports of a liquid discharge head disposed in a spaced apart relationship from a surface of the recording medium by a proper distance. An ink tank for containing the ink to be discharged is provided on a carriage together with the liquid discharge head or is disposed out of the carriage.
FIG. 5 is a view showing a schematic construction of an image forming apparatus in which the ink tank is provided on the carriage.
A carriage 101 on which a liquid discharge head (not shown) for forming an image by discharging liquid such as ink onto a recording medium 102 and an ink tank (not shown) for storing the ink to be supplied to the head are mounted is secured to a part of a belt 104 mounted between and wound around two pulleys 106a and 106b. The belt 104 is shifted by rotatingly driving one 106a of the pulleys reversibly by means of a motor 107, with the result that the carriage 101 is reciprocally shifted in directions A along a shaft 103. A signal and an electric power for causing the liquid discharge head to discharge the ink are inputted from control means (not shown) of the image forming apparatus via a cable 105.
Further, the image forming apparatus is provided with convey means (not shown) for conveying the recording medium 102 in a direction B, and the image is formed on the entire recording medium 102 by repeating a conveying operation for the recording medium 102 effected by the convey means and an image forming operation effected by the liquid discharge head mounted on the carriage reciprocally shifting with respect to the recording medium 102.
The ink tank can be exchanged with respect to the carriage 101 in accordance with consumption of the ink or new ink can be replenished into the ink tank. Depending upon kind of device of the image forming apparatus, a head cartridge in which an ink tank and a liquid discharge head are integrally formed is used so that the ink tank and the head can be exchanged integrally or collectively. In this type, since an ink storing amount is determined by a dimension of the carriage 101, it is disadvantageous when a large amount of ink is consumed.
FIG. 6 is a view showing a schematic construction of an image forming apparatus in which the ink tank is disposed out of the carriage.
An operation of the carriage 101 in the image forming apparatus shown in FIG. 6 is substantially the same as that of the carriage shown in FIG. 5. However, the image forming apparatus shown in FIG. 6 is designed so that the ink is supplied to a sub-tank 109 (refer to FIG. 7) on the carriage 101 through a tube 108 from a main tank 106 disposed out of the carriage 101.
FIG. 7 is a sectional view showing an internal construction of the carriage shown in FIG. 6.
The sub-tank 109 for storing a small amount of ink is mounted on the carriage 101, and ink flow passages are communicated from the sub-tank 109 to nozzles 110 of a liquid discharge head. The sub-tank 109 also acts as a buffer for replenishing new ink from the main tank 106 (FIG. 6) and for collecting a bubble generated in the flow passage.
As such, by designing so that the main tank 106 is disposed out of the carriage 101 and the ink is supplied to the sub-tank 109 on the carriage 101 through the tube 108, limitation regarding the dimension of the ink tank occupying the space on the carriage 101 is relaxed and, thus, the ink reserving amount of the entire apparatus can be increased. Therefore, recording regarding a recording medium 102 having a large size and recording regarding a large number of recording media 102 become possible.
However, when the ink is supplied from the main tank 106 disposed out of the carriage 101 to the liquid discharge head through the tube 108 during the recording operation, there arises a problem that pressure of the ink within the sub-tank 109 is greatly changed to affect an influence upon a discharging ability of the liquid discharge head, thereby worsening appearance quality of a recorded image. In general, it is preferable that the pressure within the sub-tank 109 is within a proper negative pressure range for balancing with discharge energy of the liquid discharge head. If the pressure is increased above such a proper range, i.e., if the pressure becomes near a positive pressure condition or it attains a positive pressure, the ink is apt to be dropped from the nozzles 110. On the other hand, if the pressure is decreased below such a proper range, i.e., if the negative pressure is further increased, the ink is hard to be discharged from the nozzles 110 and the ink discharging amount tends to be decreased.
Main factors for causing the change in pressure inherent to the case where the ink is supplied from the main tank 106 disposed out of the carriage 101 to the liquid discharge head through the tube 108 during the recording operation are (1) dynamic pressure of the tube, (2) pressure loss in the tube and (3) difference in posture.
Here, the xe2x80x9cdynamic pressure of the tubexe2x80x9d is a phenomenon that the tube 108 connected to the sub-tank 109 on the carriage 101 is vibrated as the carriage 101 is reciprocally shifted, with the result that inertia of the ink within the tube 108 acts to increase or decrease the pressure with respect to the sub-tank 109, which phenomenon is generated in accordance with acceleration/deceleration of the carriage and vibration due to such acceleration/deceleration.
Further, the xe2x80x9cpressure loss in the tubexe2x80x9d is flow resistance in the tube 108. If a consumed amount of ink due to ink discharging from the liquid discharge head becomes greater than the amount of ink supplied from the tube 108, the pressure in the sub-tank 109 will gradually be decreased, i.e., the negative pressure will be further increased. When an inner diameter of the tube 108 is sufficiently great, such a phenomenon is not noticeable, but, since the ink amount existing within the tube 108 is increased, the influence of the dynamic pressure of the tube tends to increase, contrary to the influence of the pressure loss in the tube.
Further, the xe2x80x9cdifference in posturexe2x80x9d is inclination of the installed recording apparatus. If the recording apparatus is inclined with respect to the scanning direction of the carriage 101, since at both stroke ends of the reciprocating carriage 101, height of the carriage 101 with respect to the main tank, that is, height of the nozzles 110 is differentiated, change in pressure is generated due to difference in water head.
As countermeasure for such change in pressure, in the conventional image forming apparatuses, for example, as proposed in Japanese Patent Application Laid-open No. 1-281948 (1989), a fluidity suppressing member for suppressing fluidity of ink is added to each flow passage, or, as proposed in Japanese Patent Application Laid-open No. 5-201015 (1993), a sub-tank has a pressure dampening function as a pressure damper. In the image forming apparatus, in addition to consideration of such countermeasure, effort is generally made to provide a head in which use conditions mainly including a recording speed condition are stipulated to suppress the change in pressure within the sub-tank and to permit possible pressure change.
Further, as countermeasure for dampening the influence of the pressure change of the ink, it is considered that an open/close valve is provided in a flow passage between the main tank and the sub-tank so that the valve is opened to suppress the change of pressure within the tube during the recording operation and the valve is opened to supply the ink intermittently during a waiting condition.
However, in the above-mentioned countermeasure for the change in pressure, there were the following drawbacks.
That is to say, when the ink is supplied during the recording operation, since the apparatus is designed under the condition imposed regarding the pressure change, these are determined limits to the appearance quality of the recorded image and the discharging ability, so that abilities of constructural elements of the apparatus cannot be demonstrated adequately. Further, in the apparatus in which the open/close valve is closed to suppress the pressure change during the recording operation and the valve is opened to supply the ink during the waiting condition, since a continuous recording amount is determined by the volume of the sub-tank, the merit obtained by the design in which the ink is supplied from the main tank to the sub-tank through the tube cannot be sufficiently utilized.
An object of the present invention is to provide an image forming apparatus in which change in pressure within a sub-tank caused by dynamic pressure of a tube is selectively excluded and maximum ability can be achieved in accordance with an image forming condition to which the user desires.
Another object of the present invention is to provide an image forming apparatus for forming an image by discharging liquid from a recording head onto a recording medium, comprising a carriage to which the recording head and a sub-tank for storing the liquid to be supplied to the recording head are mounted and which can be shifted reciprocally, a main tank disposed out of the carriage and adapted to store the liquid, a liquid flow passage connected between the main tank and the sub-tank in order to supply the liquid from the main tank to the sub-tank, a flow passage opening/closing valve disposed in the liquid flow passage on the way and adapted to open and close the liquid flow passage, and control means for controlling opening/closing of the flow passage opening/closing valve in accordance with an image forming condition during image forming using the recording head.